1. Field of the Invention
This invention relates to static neutralizers, sometimes commonly referred to as static-charge neutralizers. More particularly, this invention relates to low maintenance alternating-current (AC) gas flow driven static neutralizers that reduce or eliminate electrode contamination by limiting, preventing or reducing particle accumulation on the surface(s) of their respective emitter(s).
2. Background Art
A static neutralizer is commonly employed to reduce or eliminate electro-static charges that accumulate on or near electro-static sensitive items, such as flat panel displays, electronic circuits, and other items that may be damaged by the discharge of these electro-static charges. To reduce or eliminate these electro-static charges, a static neutralizer creates ions of opposite polarity, which when directed towards an area having a static charge, neutralize the static charge.
A static neutralizer creates these ions by applying a large voltage, named ionizing voltage, to at least one ion emitter, commonly referred to as an emitter or ionizing electrode. Each emitter is located in proximity to at least one reference electrode, which may be in the form of either an emitter receiving a voltage of opposite polarity or a grounded electrode. Either type of reference electrode serves to terminate the electric field from the emitter(s). Each emitter and its corresponding reference electrode(s) generate both polarity ions in the surrounding air or gas media when a sufficient voltage is maintained across the emitter and its corresponding reference electrode. An emitter and its corresponding reference electrode(s) may be referred to as an ionizing cell. This ionizing voltage produces a high voltage gradient that in turn creates an electric field near each emitter used and when this voltage exceeds the corona threshold voltage for the ionizing cell, a corona discharge results that creates ions.
The corona threshold is sometimes called the corona onset voltage for the emitter. For a wire or filament-type emitter, the corona threshold voltage is typically (+) 5-6 kV for positive and (−) 4.5-5.5 kV for negative ionizing voltages. For point-type emitters the corona onset voltage is typically 1-1.5 kV lower for both polarities. These corona onset voltage values, however, are generally applicable only to clean emitters.
It is well known in the art that an emitter accumulates particles and airborne molecular contamination from the environmental air or gas. In addition to creating ions, each emitter also are functions as an electrostatic precipitator. Attracting and collecting contamination on an emitter is a consequence of corona discharge in open air. The accumulation of contamination on an emitter changes the emitter's geometry and raises its corona onset voltage. A contaminated emitter exhibits significantly lower efficiency and disrupts the balance of generated positive and negative ions, named “ion balance”, which in turn, reduces the performance of the AC static neutralizer.
In addition, static neutralizers that apply an AC high voltage waveform with a frequency in the 103-105 Hz range to an ionizing cell sometimes suffer from high ion recombination or ion loss rates. At these frequencies, which are within the range of frequencies commonly associated with radio frequencies (RF), when the waveform of one polarity is applied to the ionizing electrode, most of the corona-generated ions of the same polarity are repelled from the electrode. Although they have enough time to move away from ionizing electrode, they cannot travel far enough to reach the low voltage or reference electrode before the waveform polarity reverses. When the polarity reverses, the same movement occurs for the other polarity of ions. Therefore a bipolar ion cloud can be formed predominantly in the central part of the gap between ionizing or ion emitting and reference electrodes. Formation of this cloud occurs for an applicable set of ion mobility, voltage amplitude and frequency values, as previously disclosed in U.S. Pat. No. 7,057,130.
These types of static neutralizers that employ a high voltage high frequency AC waveform provide a very efficient air or gas ionization and create bipolar ion clouds having high ion concentration. Electrical fields oscillating in the RF range, however, do not expel the ions and move them to the charged object. To solve this problem, these static neutralizers employ an air or gas moving means, such as a blower, fan, or compressed gas expelled through at least one nozzle, to drive these ions towards an object selected for charged neutralization.
This gas flow solution suffers from the disadvantage of increasing the rate of accumulation of unwanted particle contaminant on the emitter, such as on its body or emitter point, because of the increased airflow through the gaps in the ionizing cell. This accumulation affects emitter geometry and raises emitter corona onset voltage, which decreases real time ion production and the efficiency of the static neutralizer.
One solution includes providing clean or uncontaminated air or gas for gas flow driven static ionizers. However, this solution may be difficult or expensive to accomplish, especially in large manufacturing environments where the ionization cell is exposed to ambient air.
Another solution, as taught in U.S. Pat. Nos. 4,734,580 and 5,768,087, includes using a manual or automatic brush for cleaning the emitters of a static neutralizer. This method of mechanical cleaning is effective, but requires additional mechanical parts and, in some cases, increases emitter contamination if the manual or automatic client brush is not maintained so that it remains cleaner than the emitter being cleaned.
Another solution involves using special clean dry air (CDA) or inert gas flow (for example nitrogen) to create a protective gas sheath surrounding a tip of an emitter, which is disclosed in U.S. Pat. No. 5,847,917 and published in U.S. patent application 2006/0193100). This method is expensive and has limited application to static neutralizers that employ nozzles with pointed emitters.
FIG. 1 shows a schematic view of a known DC static neutralizer 2 that generates a bipolar ion cloud (not shown) and which is disclosed in U.S. Pat. Nos. 5,055,963 and 6,118,645 and published U.S. patent application 2003/0218855. This type of system requires two very stable high voltage DC power supplies 4a and 4b that separately provide ionizing voltages 6a and 6b, which are of different polarities at constant voltage magnitudes +U and −U, to at least two emitters 8a and 8b, and as such, is relatively costly to manufacture and maintain. This type of DC static neutralizer suffers from a relatively high contamination because airborne particles become charged as they approach the ionizing cell 10 and are continuously attracted to the positive and negative emitters 8a and 8b since they continuously receive their respective ionizing voltages.
FIG. 2 shows a schematic view of a pulsed DC static neutralizer 12 disclosed in U.S. Pat. Nos. 3,711,743; 4,901,194; and 4,951,172. Pulsed DC neutralizer 12 is similar to DC neutralizer 2 but uses a positive power supply 14a and a negative power supply 14b that respectively provide output waveforms 15a and 15b to separate emitters 18a and 18b, sometimes referred to as ionizing electrodes. Output waveforms 15a and 15b are waveforms that respectively have pulsed ionizing voltages 16a and 16b, as shown. This type of DC neutralizer has a relatively low ion recombination rate but suffers from a relatively high emitter contamination rate and system complexity.
FIG. 3 illustrates another example of a pulsed DC static neutralizer 20, which is further disclosed in Japanese patent JP2004039352 and U.S. patent application 2005/0116167, that uses positive and negative high voltage power supplies 21a and 21b that are periodically switched by a microprocessor (not shown), and their respective two voltages and combined in a summing circuit 22. This low frequency system uses only one high voltage bus 24 for sending the output 25 of summing circuit to all ion emitters, including emitter 26. The rate of accumulation of contaminants on these emitters is approximately the same as for pulsed DC systems, such as pulsed DC neutralizer 12. The output waveforms disclosed in FIGS. 1 and 3 use either DC or slowly or slowly switched DC pulses of less than 5 Hz.
As illustrated in FIG. 4 and disclosed in U.S. Pat. No. 4,757,422 and patent application 2005/0286201, many AC static neutralizers, such as static neutralizer 28 employ a simple line frequency (50-60 Hz) step-up transformer 30 as its high voltage power supply, and typically use a low frequency ionizing output waveform 32 of around 100 Hz or less. These AC static neutralizers are inexpensive, but because of the low frequency ionizing voltage, the step-up transformers are quite large, rendering these static neutralizers bulky. In addition, these types of AC static neutralizers have a contamination rate in excess of pulsed DC neutralizers, such as neutralizers 12 and 20, above.
FIG. 5 illustrates another example of a gas flow-driven AC static neutralizer 34, which is further disclosed in U.S. Pat. No. 6,646,856 and in Japanese patent JP 2004273357. Static neutralizer 34 is shown with two emitters 35a and 35b per ionization cell 36. Emitters 35a and 35b receive a high frequency continuous output waveform 37 from a high voltage power supply 38 that has an amplitude sufficient for positive and negative ion generation by corona. This amplitude has a maximum peak-to-peak magnitude that remains fixed and does not vary over time. Static neutralizer 34 also includes an air blower (not shown), and its power supply 37 may be manufactured inexpensively and at a relatively small foot print. Static neutralizer 34, however, suffers from a relatively high contamination rate because its emitters require cleaning approximately every 50 to 100 hours of operation.
Consequently, a need exists for a low maintenance AC gas flow driven static neutralizer that limits, prevents or reduces the accumulation of gas borne contamination particles on its emitter(s).